When filling an empty container, it is sometimes easy to overlook the fact that it is, in fact, already full. However, it is full of gas, not liquid.
A typical filling machine must manage this gas. When filling a container, the liquid displaces the gas. This gas must be disposed of as it leaves the container. In addition, the air that normally fills an empty container has oxygen. This is often harmful to products. As such, it is often useful to flush the container with some inert gas to displace oxygen-laden air. In some cases, it is useful to evacuate the container, so that it truly is empty, or as nearly empty as it is possible to make it. In yet other cases, it is useful to prepare the container to receive liquid by pre-tensioning it with a high pressure pre-tensioning gas
A modern filling machine has one or more gas paths to accomplish one or more of these tasks. The need to control these gas paths introduces considerable complexity into the design of a filling machine.